Aer Lingus chief executive Christoph Mueller has been criticised for trying to delay a report into a row over a €780m pension scheme deficit for more than 14,000 aviation workers.

SIPTU, which represents 2,800 Dublin Airport Authority and Aer Lingus staff, also warned that it would ballot for industrial action if the expert report, due for release on Monday, was rejected by its members.

It is unclear when the report by the Expert Panel on the Irish Airlines Superannuation Scheme (IASS) will be released, but sources said it was still likely to go ahead on Monday.

The Department of Transport said it could not indicate when it will be issued.

Transport Minister Leo Varadkar set up the Expert Panel in March to resolve a row over the deficit after failed interventions by the Labour Relations Commission and Labour Court. The scheme covers current and former workers at Dublin Airport Authority, Aer Lingus, Shannon Airport Authority and former SR Technics staff.

Mr Mueller urged the panel to hold off issuing the report as he was extremely concerned about its draft proposals. He said he feared they may not be accepted by shareholders.

He is already under intense pressure after the airline lost up to €10m due to a strike by cabin crew last month. Aer Lingus has earmarked €110m to shore up the pension deficit for staff, with another €30m for deferred members, who are former staff who have not yet retired. But unions say these amounts are not enough.

Sources believe the report will recommend that the airline pump even more cash into the scheme, with Aer Lingus footing a bill up to €200m.

In his letter to former IBEC director Brendan McGinty, a member of the panel, Mr Mueller asked that the burden be 'fairly' split with the DAA. SIPTU, which has served notice of industrial action over the pensions row, branded Mr Mueller's intervention as "totally unacceptable".

"If the chief executive of Aer Lingus is making last-minute interventions, it is totally unacceptable to us," said SIPTU national pensions policy adviser Dermot O'Loughlin. "We gave our last and final comments to the panel, and were led to believe everyone else had."

He added: "Therefore, if it was to be delayed, that would clearly indicate someone was trying to influence the process for their vested interests."

He said an injunction granted to Dublin Airport Authority (DAA) to halt strikes last March would not prevent further ballots.

A four-hour strike was averted in March just ahead of the St Patrick's weekend when the High Court granted an injunction to DAA to halt the action, while Aer Lingus is suing Mr O'Loughlin for alleged damages to business.